Aluminium is the most widely used non-ferrous metal. Although it is one of the most abundant elements, Aluminium in its native state is rare and instead it is primarily converted from Aluminium oxide (Al2O3), which is also known as “Alumina”. Aluminium oxide is largely produced or extracted from aluminium ores, primarily from the aluminium ore identified as Bauxite. Aluminum hydroxide production processes are used for extraction and production of the alumina, or aluminium oxide, from the Bauxite, which also typically contains of silica, various iron oxides, and titanium dioxide. After the aluminium oxide is purified, it is thereafter refined to produce aluminium metal. The aluminium hydroxide product process is the largest single cost in connection with aluminium production.
The principle industrial scale means of refining bauxite and producing aluminum hydroxide is by the well-established methods of the Bayer process. The Bayer process typically comprises a digestion stage, wherein alumina is extracted by digesting the bauxite ore in a solution of sodium hydroxide under high pressure and temperature, forming soluble sodium aluminate; followed by a clarification stage, wherein solid phase residue, known as “red mud”, is removed from the sodium aluminate in solution; thereafter a precipitation stage, wherein aluminum hydroxide is precipitated from the sodium aluminate solution and grown in the form of aluminum hydroxide crystals, typically using seeding methods; followed by a classification stage, wherein crystal seeds are separated from the aluminum hydroxide product material; and thereafter a calcination stage, wherein the aluminium hydroxide decomposes to aluminium oxide, the alumina end product.
In order to increase the yield of aluminum hydroxide (Al(OH)3) from the aluminate process liquors and achieve a given crystal size distribution, precipitation process operators carefully control operating parameters, such as precipitation temperature, cooling rate and seeding. Seeding, as described in European Patent Specification EP 0465055B1, is conducted in the precipitation stage and involves adding seed material to liquor pregnant with alumina. The seeds function as surface sites, aiding in the nucleation and growth of alumina bearing crystals of a particular distribution of specific sizes. In particular, some crystals are targeted as having a size optimized to facilitate easy and efficient separation from the liquor and further processing. Other crystals are targeted to having a size optimized for functioning as future seeds.
Even with such methods of controlling operating parameters, production is often limited because increasing yield solely by adjusting plant conditions can decrease particle size and in that parameters can vary from one plant to the next. Such parameters can include, but are not limited to, temperature profiles, seed charge, seed crystal surface area, purge of carbon dioxide or flue gases, liquor loading, liquor purity, and the like.
Extensive efforts have been invested into finding chemical additives and methods limiting the factors negatively affecting particle size in order to achieve the optimal economic recovery of aluminum hydroxide product. Such efforts include adding crystal growth modifiers (CGM) in the precipitation stage as an avenue to improve the yield of aluminum hydroxide product, while maintaining particle size. However, despite the continuous and ongoing development of methods suitable for obtaining aluminum hydroxide crystals with increased particle size, there is still a desire for improvements and enhancements for the aluminium hydroxide production process to address production quality and economic concerns.
The art described in this section is not intended to constitute an admission that any patent, publication or other information referred to herein is “prior art” with respect to this invention, unless specifically designated as such. In addition, this section should not be construed to mean that a search has been made or that no other pertinent information as defined in 37 CFR §1.56(a) exists.